Thats all isnt an excuse to lock covenant abilities.
If you're roleplayer and want to lock it for yourself - do not change it till the end of xpac, nothing prevents you. If you're minmaxer - swap it every trash pack. And anything between it. That simple.
I want to chose the way I want to play. If I want to lock class and min-max abilites - I can do it. If I want to change class every balance patch - I can do it.
If I want to change covenant ability - uh-oh, nopenope, that ruin the game!!!11
99% of groups doesnt need your "unique utilities", they just want to end it quicker.
Also, speaking of roleplaying. While I'm a sort of raider\m+, I have some time roleplaying with some friends. And I'm actually limited in roleplay because of lacking ability to change class\faction. While we played a story back in WotLK my paladin needed to become a deathknight, so I was forced to delete paladin and level up a deathknight. And another our story stopped because we needed alliance blood elf for it, something like a deserter or sort of it. Waiting when blizz allows us to change faction without changing race Warcraft lore allows it, but regretfully not the game.